The Epston NX510 cartridges I received do not agree with either of your photos. I have a tear drop shaped plug at one end, a small round button at the other. Your photos show one case with tear drop plug as Air Vent". The other photo shows the tear drop plug as "Refill hole". Which is which? Does it matter? A simple one-sheet instruction shipped with the kit would be a great help and not too expensive!

I'm sorry that the pictures didn't match up to the item you received and for the late reply. I believe that we took care of this situation with you over the phone.

In situations like this, it is best to give us a call directly so we can help you over the phone. If you received a cartridge with two plugs, there is a 'general' trick to figure out which plug belongs to to the 'air vent' hole and has to be removed. If you look right below each plug, there should be only one plug that has a 1/3 inch 'internal square compartment' right underneath it. This plug would the the 'air vent' plug.